How To Create Number Field With Dropdown Field In Flutter

dependencies:
  flutter:
    sdk: flutter
  flutter_masked_text: ^0.8.0

flutter pub get

import 'package:flutter_masked_text/flutter_masked_text.dart';

import 'package:flutter/material.dart';
import 'package:flutter_masked_text/flutter_masked_text.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}
class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
//  static final List<String> _dropdownItems = <String>['India', 'USA'];
  static List<CountryModel> _dropdownItems = new List();
  final formKey = new GlobalKey<FormState>();

  var controller = new MaskedTextController(mask: '(000) 000 0000');
  CountryModel _dropdownValue;
  String _errorText;
  TextEditingController phoneController = new TextEditingController();

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    setState(() {
      _dropdownItems.add(CountryModel(country: 'India', countryCode: '+91'));
      _dropdownItems.add(CountryModel(country: 'USA', countryCode: '+1'));
      _dropdownValue = _dropdownItems[0];
      phoneController.text = _dropdownValue.countryCode;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.amber,
        title: Text('Number Field '),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(15.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              _buildCountry(),
              SizedBox(height: 10),
              _buildPhonefiled(),
            ],
          ),
        ),
      ),
    );
  }

  Widget _buildCountry() {
    return FormField(
      builder: (FormFieldState state) {
        return DropdownButtonHideUnderline(
          child: new  Container(
            child: FormField(
              builder: (FormFieldState state) {
                return DropdownButtonHideUnderline(
                  child: new Column(
                    crossAxisAlignment: CrossAxisAlignment.stretch,
                    children: <Widget>[
                      Padding(
                        padding: const EdgeInsets.all(0.0),
                        child: new InputDecorator(
                          decoration: InputDecoration(
                            enabledBorder: OutlineInputBorder(
                              borderRadius: BorderRadius.all(Radius.circular(12.0)),
                              borderSide: BorderSide(color: Colors.black26),
                            ),
                            focusedBorder: OutlineInputBorder(
                              borderRadius: BorderRadius.all(Radius.circular(12.0)),
                              borderSide: BorderSide(color: Colors.grey),
                            ),
                            contentPadding: EdgeInsets.all(7.0),
                            filled: false,
                            prefixIcon: Icon(Icons.location_on,size: 15, color: Colors.black45,),
                            hintText: 'Choose Country',
                            hintStyle: TextStyle(color: Colors.black45,fontSize: 12),
                            labelText:
                            _dropdownValue == null ? 'Where are you from' : '',
                            errorText: _errorText,
                          ),
                          isEmpty: _dropdownValue == null,
                          child: new DropdownButton<CountryModel>(
                            value: _dropdownValue,
                            isDense: true,
                            onChanged: (CountryModel newValue) {
                              print('value change');
                              print(newValue);
                              setState(() {
                                _dropdownValue = newValue;
                                phoneController.text = _dropdownValue.countryCode;
                              });
                            },
                            items: _dropdownItems.map((CountryModel value) {
                              return DropdownMenuItem<CountryModel>(
                                value: value,
                                child: Text(value.country,
                                    style: TextStyle(
                                        color: Colors.black45,
                                        fontSize: 12
                                    )),
                              );
                            }).toList(),
                          ),
                        ),
                      ),
                    ],
                  ),
                );
              },
            ),
          ),
        );
      },
    );
  }

  Widget _buildPhonefiled() {
    return Row(
      children: <Widget>[
        new Expanded(
          child: new TextFormField(
            controller: controller,
            keyboardType: TextInputType.number,
            decoration: InputDecoration(
              enabledBorder: OutlineInputBorder(
                borderRadius: BorderRadius.all(Radius.circular(12.0)),
                borderSide: BorderSide(color: Colors.black26),
              ),
              focusedBorder: OutlineInputBorder(
                borderRadius: BorderRadius.all(Radius.circular(12.0)),
                borderSide: BorderSide(color: Colors.grey),
              ),
              contentPadding: EdgeInsets.all(15),

              filled: false,
              labelText: 'mobile',
              labelStyle: TextStyle(fontSize: 10,color: Colors.black45),
              hintText: "Mobile number",
              hintStyle: TextStyle(fontSize: 12,color: Colors.black45),
              prefixIcon: new Icon(Icons.mobile_screen_share),
            ),
            onSaved: (String value) {},
          ),
          flex: 5,
        ),
      ],
    );
  }
}

class CountryModel {
  String country = '';
  String countryCode = '';

  CountryModel({
    this.country,
    this.countryCode,
  });
}